It has been the general practice to use piezoelectric substrates having large electromechanical coupling coefficients, such as lithium niobate (LiNbO3) substrates, in order to produce surface acoustic wave filters featuring wideband characteristics.
However, the surface acoustic wave filters using substrates of such kind have a drawback in general that they are poor in temperature characteristic.
To this end, patent document 1 discloses a structure contrived to improve the temperature characteristic.
The structure disclosed in the patent document 1 comprises a LiNbO3 substrate and a thin layer of SiO2 film formed on the substrate, wherein the LiNbO3 substrate is a rotated Y-cut substrate having a cut angle set to a range of −10 to +30 degrees, and further wherein, when a film thickness of the thin layer and a wavelength of a center operating frequency of the surface acoustic wave are denoted by H and λ respectively, a value of H/λ is set to a range of 0.115 to 0.31.
Surface acoustic wave resonators formed on such a substrate are then coupled into a ladder configuration to complete a surface acoustic wave filter having a wide-band characteristic.
FIG. 15A is a schematic view of a conventional surface acoustic wave resonator, and FIG. 15B is a sectional view taken along the line A-A.
As shown in FIGS. 15A and 15B, the conventional surface acoustic wave resonator has a structure, in which comb electrodes 1502 and reflector electrodes 1503 are formed on piezoelectric substrate 1501, and a thin SiO2 film 1504 is formed over them.
The surface acoustic wave resonator constructed as above has characteristic curves shown in FIGS. 16A and 16B. FIG. 16A shows a pass-band characteristic and FIG. 16B shows admittance (Y11). In this instance here, the piezoelectric substrate is a rotated Y-cut substrate of LiNbO3 having a cut angle of 5 degrees, and the electrodes and the thin SiO2 film have thicknesses of values standardized according to a wavelength, which are 8% and 20% respectively. The electrodes are composed of a material containing aluminum as a principal component. There appear a plurality of spurious responses in the surface acoustic wave resonator as shown in these figures.
There are cases that spurious responses occur due to resonances in the transverse mode when this type of substrate is used. A method generally employed to suppress the spurious responses is to assign weights on the comb electrodes.
FIG. 17 shows a schematic view of a surface acoustic wave resonator having comb electrodes weighted by apodization for suppression of the spurious responses.
As shown in FIG. 17, comb electrodes 1701 have a shape weighted by apodization in a manner that their interdigitating lengths decrease from the center toward both sides thereof.
Characteristic curves of this surface acoustic wave resonator are shown in FIGS. 18A and 18B. FIG. 18A shows a pass-band characteristic and FIG. 18B shows admittance.
It is apparent in the characteristics of the surface acoustic wave resonator shown in FIGS. 18A and 18B that the spurious responses attributed to resonances in the transverse mode are suppressed as compared to those of FIGS. 16A and 16B.
In this type of surface acoustic wave resonator, however, there occur insuppressible spurious responses on the lower side of a resonance frequency that are considered to be attributable to the Rayleigh mode propagation, although the spurious responses by the resonances in the transverse mode are suppressed.
Levels of spurious response 1801 in the pass-band characteristic and spurious response 1802 in the admittance are as large in values as 0.2 dB and 1.0 dB respectively, as shown in FIGS. 18A and 18B. These types of conventional surface acoustic wave resonators thus have a drawback that ripples present within the frequency band adversely affect the characteristics when they are used for a ladder type filter or an antenna duplexer.    [Patent Document 1] Japanese Patent Unexamined Publication, No. 2003-209458